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Acid corrosion (not resulting from the presence of carbon dioxide) can also occur in boilers. When condenser leakage occurs or boiler feedwater becomes contaminated with carryover from the makeup evaporator, one of the major salts that is introduced to the boiler water is magnesium chloride. The composition of seawater was shown in Section I. Magnesium is the second most plentiful cation in seawater and chlorides are the most plentiful anion.

When magnesium chloride enters high-temperature boiler water, the magnesium ions react with the phosphates and hydroxyl ions in the boiler water. Magnesium ions are so reactive with hydroxyl ions at high temperature in boiler water that magnesium hydroxide precipitation will occur until the boiler water pH drops to approximate pH = 4.0. This can be prevented by proper boiler water treatment. When the pH value is not carefully controlled, hydroxyl ion concentrations are reduced to low levels. This results in hydrochloric acid attack to the metal surfaces.

MgCl2 + 2H2O Heat--> Mg(OH)2 + 2HCI

and

Fe++ +2HCl Heat--> Fe(Cl)2+H2

The presence of deposits on boiler tube surfaces can lead to concentration of hydrochloric acid underneath the deposits. When this occurs the corrosion rates become extremely high and serious damage can occur in a very short time.

Related Terms

CORROSION INHIBITOR

Additive for protecting lubricated metal surfaces against chemical attack by water or other contaminants. There are several types of corrosion inhibitors. Polar compounds wet the metal surface preferentially, protecting it with a film of oil. Other compounds may absorb water by incorporating it in a water-in-oil emulsion so that only the oil touches the metal surface. Another type of corrosion inhibitor combines chemically with the metal to present a non- reactive surface.

False brinelling

False brinelling of needle roller bearings is actually a fretting corrosion of the surface since the rollers are the I.D. of the bearing. Although its appearance is similar to that of brinelling, false brinelling is characterized by attrition of the steel, and the load on the bearing is less than that required to produce the resulting impression. It is the result of a combination of mechanical and chemical action that is not completely understood, and occurs when a small relative motion or vibration is accompanied by some loading, in the presence of oxygen.

INHIBITOR

A substance that slows or prevents such chemical reactions as corrosion or oxidation.

FRETTING CORROSION

Can take place when two metals are held in contact and subjected to repeated small sliding, relative motions. Other names for this type of corrosion include wear oxidation, friction oxidation, chafing, and brinelling.

SCC

Stress corrosion cracking.

FILIFORM CORROSION

Corrosion that occurs under organic coatings on metals as fine wavy hairlines.

CAVITATION CORROSION

A process involving conjoint corrosion and cavitation.

PIT

Corrosion localized in a small spot.

CHEMICAL CLEANING

Using a solvent solution to remove mill scale and corrosion products.

DEPOSIT CORROSION

Corrosion occurring under or around a discontinuous deposit on a metallic surface (also called poultice corrosion).

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